The invention relates to a method for controlling a converter with at least two phase modules having an upper and a lower valve branch having in each case two two-pole subsystems connected in series at low output frequencies.
Such a converter with distributed energy stores is known from the publication “Modulares Stromrichterkonzept für Netzkupplungsanwendung bei hohen Spannungen”, by Rainer Marquardt, Anton Lesnicar and Jüml urgen Hildinger” [Modular Converter Concept for System Coupling Application at High Voltages], printed in the conference proceedings of the ETG Conference 2002. In this publication, such a converter is used for a system-side and load-side converter, with these two converters being coupled to one another with distributed energy stores on the DC-voltage side.
FIG. 1 shows in more detail such a converter with distributed energy stores. In accordance with this circuit arrangement, this known converter circuit has three phase modules, which are each denoted by 100. These phase modules 100 are connected electrically conductively on the DC-voltage side in each case to a connection P or N with a positive or negative DC voltage busbar P0 or N0. A DC voltage Ud is present between these two DC voltage busbars P0 and N0. Each phase module 100 has an upper and a lower valve branch T1 or T3 or T5 and 12 or T4 or T6. Each of these valve branches T1 to T6 has a number of two-pole subsystems 10 which are connected electrically in series. In this equivalent circuit diagram, four subsystems 10 are illustrated per valve branch T1, . . . , T6. Each node between two valve branches T1 and T2 or T3 and T4 or T5 and T6 of a phase module 100 forms a connection L1 or L2 or L3 of this phase module 100 on the AC-voltage side.
FIG. 2 shows in more detail an embodiment of a known two-pole subsystem 10. The circuit arrangement shown in FIG. 3 represents a functional equivalent variant. These two subsystems 10 and 11 are described in more detail in DE 101 03 031 A1, which laid-open specification also describes the way in which said subsystems operate.
A further embodiment of a two-pole subsystem 20 is shown in more detail in FIG. 4. This embodiment of the two-pole subsystem 20 is known from DE 10 2005 041 087 A1. The design of this two-pole subsystem 20 and the way in which it operates are described in detail in this laid-open specification, and therefore no explanation in relation to this is necessary at this juncture.
The number of independent energy stores 9 and 29, 30 which are connected in series between a positive connection P and a connection L1 or L2 or L3 of a phase module 100 on the AC-voltage side is referred to as the series operating cycle n. It is advantageous here, but not absolutely necessary, to implement the same series operating cycle n between a connection L1 or L2 or L3 on the AC-voltage side and a negative connection N of a phase module 100. As shown in FIG. 1, each valve branch T1, . . . , T6 of the polyphase converter has four two-pole subsystems 10, which are connected electrically in series. Since these subsystems 10 each have only one independent energy store 9, a series operating cycle of n=4 results. If, instead of these subsystems 10, four subsystems 20 are used as shown in FIG. 2, this results in a series operating cycle n=8 since each subsystem 20 has two independent energy stores 29 and 30.
For the following explanation it is assumed that all of the energy stores 9 of the subsystems 10 of each valve branch T1, . . . , T6 of this polyphase converter are each charged to the same voltage Uc. A method for charging this energy store 9 is described, for example, in the conference proceedings for the ETG Conference 2002.
The voltages u1(t), . . . , u6(t) at the valve branches T1, . . . , T6, also referred to as valve branch voltage u1(t), . . . , u(t), comprise a DC variable ½Ud and an AC voltage variable u10(t), u20(t), u30(t). This AC voltage variable u10(t) or u20(t) or u30(t) has, firstly a frequency and an amplitude of a desired output voltage of the converter. These AC variables u10(t), u20(t) and u30(t) are related to a fictitious mid-point 0 between the two DC voltage busbars P0 and N0, as shown in FIG. 1. This results in sinusoidal converter output voltages u10(t), u20(t) and u30(t), wherein the following must apply for the amplitudes of the voltages u10(t), u20(t) and u30(t) related to the mid-point 0: each amplitude of an AC voltage variable u10(t), u20(t) and u30(t) should always be less than half the DC voltage Ud. The voltage u1(t) or u2(t) or u3(t) or u4(t) or u5(t) or u6(t) of a valve branch T1 or T2 or T3 or T4 or T5 or T6 must therefore always be positive since all of the two-pole subsystems 10 of a valve branch T1, . . . , T6 which are connected in series can generate only a short circuit or a positive voltage at the output terminals X1 and X2 of each two-pole subsystem 10, irrespective of the valve branch current direction in all switching states. Owing to the structure of these two-pole subsystems 10, 11 and 20, negative voltages are not possible. Therefore, the valve voltage u1(t) or u2(t) or u3(t) or u4(t) or u5(t) or u6(t) of each valve branch T1 or T2 or 13 or T4 or 15 or 16 can vary between zero and n times a capacitor voltage Uc of the n independent energy stores 9 and, respectively, 29, 30.
FIG. 5 shows a characteristic of the valve branch voltage u1(t) and of the valve branch current i1(t) of the valve branch T1 of the phase module 100 of the polyphase converter shown in FIG. 1 in a graph over time t. If the two characteristics are multiplied by one another, the time characteristic of an instantaneous power PT1(t) of this valve branch T1 is produced, which is illustrated in a graph over time t in FIG. 6. If this instantaneous power PT1(t) of the valve branch T1 is integrated over a period of the valve branch voltage u1(t) (corresponds to the areas below the curved sections of the curve of the instantaneous power PT1(t)), in the steady state the value zero is always reached. This means that the energy stores 9 of the two-pole subsystems 10 in this valve branch T1 in total do not receive or emit any energy. The same also applies to all of the other valve branches T2, . . . ,T6 of the polyphase converter shown in FIG. 1.
It follows from this that the energy content of each energy store 9 of each valve branch T1, . . . , T6 of the polyphase converter shown in FIG. 1 and therefore of this polyphase converter is constant in the steady state. For this reason, these two-pole subsystems 10 and 11 and 20 also do not require an active power feed to the respective DC voltage connections of the energy stores 9 and 29, 30, respectively.
An energy content of each energy store 9 or 29, 30 of the two-pole subsystems 10, 11 and 20, respectively, of each valve branch T1, . . . , T6 is advantageously dimensioned in accordance with the maximum required energy deviation. It is necessary here to take into account the fact that the voltage ripple ΔU which is superimposed on the steady-state voltage mean value in the energy stores 9 and 29,30 should not overshoot a maximum predetermined limit value. This maximum voltage is determined by the dielectric strength of the semiconductor switches and energy stores 9 and 29, 30 which can be switched off and are used in the two-pole subsystems 10, 11 and 20, respectively, and also by means of regulation technology. A decisive factor in the dimensioning of the energy stores 9 and 29, 30 is the output frequency of the polyphase converter shown in FIG. 1. The lower this output frequency is, the greater the energy deviation is per period in the energy store 9 or 29, 30. This means that, for a predetermined voltage ripple ΔU, the required variable of the energy stores 9 and 29, 30 of the two-pole subsystems 10, 11 and 20, respectively, would tend towards infinity in hyperbolic fashion as the frequency decreases up to the DC voltage operating mode (frequency equal to zero).
This relationship between the voltage ripple ΔU and the output frequency f of the polyphase converter shown in FIG. 1 is illustrated in a graph shown in FIG. 7. This graph shows a hyperbolic curve A for the voltage ripple of an energy store (continuous line) and a hyperbolic curve B for the voltage ripple when using three partial energy stores in parallel per energy store 9 or 29, 30, i.e. three times the intermediate-circuit capacitance (dashed line). The hyperbolic curve A shows that, starting from an output frequency f=50 Hz, the voltage ripple ΔU increases substantially as the frequency decreases. If at half the output frequency the voltage ripple ΔU should be equal to the voltage ripple ΔU at the output frequency f=50 Hz, the value of an energy store 9 or 29, 30 of a two-pole subsystem 10, 11 or 20 must be a multiple greater.
The graph in FIG. 8 shows a characteristic of the valve branch voltage u1(t) with an output frequency f=50 Hz and a characteristic of this valve branch voltage u1(t) at an output frequency of f=5 Hz over time t. The amplitude of the valve branch voltage u1(t) at an output frequency f=5 Hz has been decreased corresponding to a u/f characteristic. If a recalculation is performed taking into consideration the corresponding valve branch current in the valve branch T1 of the polyphase converter shown in FIG. 1, an associated instantaneous power PT1(t) at an output frequency f=50 Hz and f=5 Hz is produced. These two characteristics of the instantaneous power PT1(t) of the valve branch T1 are shown in the graph in FIG. 9 over time t. The energy deviation at the output frequency f=5 Hz has risen substantially in comparison with the energy deviation at the output frequency f=50 Hz. In this example illustrated, the energy deviation at f=5 Hz is 25 times greater than at f=50 Hz.
In order to produce the same voltage ripple ΔU as at the output frequency f=50 Hz in this operating point as well (f=5 Hz), the energy store 9 or 29, of the two-pole subsystems 10, 11 or 20 would need to be dimensioned to be a factor of 25 greater.
In order to arrive at a solution which is attractive in terms of size and costs, it is advantageous if the design of the energy stores 9 and 29, 30 of the two-pole subsystems 10, 11 and 20, respectively, of the valve branches T1, . . . , T6 of the polyphase converter shown in FIG. 1 is performed for a rated working point. This means that, in this rated working point, the energy deviation already results in a predetermined maximum permissible voltage ripple ΔU. For operation at low frequencies, i.e. below a rated frequency fN, up to purely DC operation (f=0 Hz), as arises when running up drives, the control methods in accordance with the prior art cannot be used for a realistic and competitive design of the energy stores 9 and 29, 30 of two-pole subsystems 10, 11 and 20 used.